release: Fix powerpc boot block uudecoding

Confusingly, uuencode writes to stdout by default, but uudecode does not.

Fixes: 6c3e01bf8578 "release: Randomize powerpc boot block file name"
MFC after: 3 days

(cherry picked from commit 4d7174b1d91ff3651ef78987d93989a40a4facd2)